Conventionally, as a wire drawing die for drawing an ultra fine wire with a hole diameter of 50 μm or less, the one having a shape as shown in FIG. 1 made of natural monocrystalline diamond or synthetic monocrystalline diamond has been used. However, monocrystalline diamond has a problem that, when it is used for wire drawing for a long period of time, uneven wear as shown FIG. 2B occurs and a wire surface is deteriorated. In diamond monocrystal, distances between crystal lattice planes differ depending on orientation, and the lattice planes have different in-plane atomic densities. Therefore, diamond monocrystal has wear resistance that is significantly direction-dependent, which causes uneven wear after wire drawing and deterioration in the wire surface.
Further, a die for drawing a highly hard wire such as a stainless wire, a steel cord, or the like has a problem that an excess stress is exerted on the die during wire drawing and a crack due to cleavage occurs. Therefore, polycrystalline diamond is generally used at present for such an application.
Currently, all polycrystalline diamonds marketed for use in tools use an iron group metal such as Co, Ni, Fe, or a ceramic such as SiC, as a sintering aid or a binding agent. They are obtained by sintering diamond powder together with a sintering aid or a binding agent under high-pressure and high-temperature conditions in which diamond is thermodynamically stable (generally, at a pressure of 5 to 6 GPa and at a temperature of 1300 to 1500° C.). However, since they contain around 10% by volume of a sintering aid or a binding agent, it is not possible to obtain a highly precise hole surface, and thus such a polycrystalline diamond is not applicable to ultra fine wire drawing. Although naturally produced polycrystalline diamonds (carbonado and ballas) are also known, and some of them are used as a drill bit, they have many defects and they considerably vary in material quality. Therefore, they are not used for the application as a die.
On the other hand, a polycrystalline body of single phase diamond having no binding agent is obtained by directly converting non-diamond carbon such as graphite, glassy carbon, amorphous carbon, or the like into diamond and simultaneously sintering the diamond at an ultra high pressure and an ultra high temperature without a catalyst or a solvent.
As such a polycrystalline body, for example, J. Chem. Phys., 38 (1963) 631-643 [F. P. Bundy] (Non-Patent Document 1), Japan. J. Appl. Phys., 11 (1972) 578-590 [M. Wakatsuki, K. Ichinose, T. Aoki] (Non-Patent Document 2), and Nature 259 (1976) 38 [S. Naka, K. Horii, Y. Takeda, T. Hanawa] (Non-Patent Document 3) disclose obtaining polycrystalline diamond by subjecting graphite as a starting material to direct conversion at an ultra high pressure of 14 to 18 GPa and an ultra high temperature of 3000 K or more.
Further, Japanese Patent Laying-Open No. 2002-066302 (Patent Document 1) describes a method of synthesizing fine diamond by heating carbon nanotube to 10 GPa or more and 1600° C. or more.
Furthermore, New Diamond and Frontier Carbon Technology, 14 (2004) 313 [T. Irifine, H. Sumiya] (Non-Patent Document 4) and SEI Technical Review 165 (2004) 68 [Sumiya, Irifune] (Non-Patent Document 5) disclose a method of obtaining dense and highly pure polycrystalline diamond by subjecting highly pure graphite as a starting material to direct conversion and sintering by indirect heating at an ultra high pressure of 12 GPa or more and an ultra high temperature of 2200° C. or more.    Patent Document 1: Japanese Patent Laying-Open No. 2002-066302    Non-Patent Document 1: J. Chem. Phys., 38 (1963) 631-643 [F. P. Bundy]    Non-Patent Document 2: Japan. J. Appl. Phys., 11 (1972)578-590 [M. Wakatsuki, K. Ichinose, T. Aoki]    Non-Patent Document 3: Nature 259 (1976)38 [S. Naka, K. Horii, Y. Takeda, T. Hanawa]    Non-Patent Document 4: New Diamond and Frontier Carbon Technology, 14 (2004) 313 [T. Irifune, H. Sumiya]    Non-Patent Document 5: SEI Technical Review 165 (2004) 68 [Sumiya, Irifune]